Defects such as corrosion, cracks etc weaken the structural integrity of a material or structure. The failure of a structure in many industries may have very serious consequences such as the failure of a structure forming a component of a pipeline, such as a gas or oil pipeline or an aircraft panel. Consequently, considerable effort is directed to the inspection of structures to determine whether they have or acquire any defects which may affect their structural integrity.
Structures can be visually inspected in an attempt to detect defects. However, whilst some defects may be able to be detected visually, such as serious corrosion which may result in surface distortion, many defects, such as cracks or corrosion which is not easily visible from the outside surface of the structure may be missed. Furthermore, many structures are covered in layers of paint, coatings and insulation which would make visual inspection even more difficult.
Another method of inspecting structures to detect defects involves ultrasonic waves. However, ultrasonic waves can only penetrate a single layer of a structure unless subsequent layers are mechanically bonded together. Thus, defects in multilayer structures are difficult to detect.
Another method of detecting defects in structures uses eddy currents. Eddy current inspection offers important advantages for the detection of flaws in conductive structures and has a very broad range of applications. It is non-destructive, and can detect flaws invisible to the unaided eye. A single frequency is commonly used to realize non-destructive inspection. Sometimes two frequencies or multi frequencies may be used in order to improve inspection capability and detection reliability. Eddy currents can penetrate multi-layers of material to discover hidden damage which may threaten the serviceability of a material or structure. However, for precisely locating a defect position in a complex structure, conventional eddy current techniques encounter difficulties caused by frequency attenuation. A Pulsed Eddy Current (PEC) square wave pulse provides wide band frequency energy, particularly at low frequencies. Therefore it provides more information regarding defect depth and the potential for subsequent data processing.
Probe lift-off is a problem associated with passing a probe over a surface of a structure to be tested in that variations in the spacing between the probe and the structure to be inspected produce false variations in the received signal which may mask variations indicative of real defects which is misleading for the identification of defects. The variations in the lift-off may be produced, for example, by variations in the structure surface over which a probe is passed such as paint and/or coating thickness, the presence of surface features such as rivets and the variation in pressure applied to the probe by the operator when in use.
U.S. Pat. No. 6,344,741 and CA 2 312 101 disclose a method for compensating for lift-off variations in pulsed eddy current techniques. Calibration signals are obtained from a probe at two or more different lift-off distances from a structure to be tested at a position free from defects. The point where the detected signals cross in the time domain is determined to establish a “lift-off point of intersection” at that particular point in time of the time domain signal. The amplitudes of subsequent time domain test signals are compared at that same point of time at which the calibration signals crossed to compensate for lift-off.
However, the intersection point may not be very precise and may drift when different size defects are detected. Furthermore, lift-off compensation is only achieved at a single point on the received signal such that only a limited amount of information is available from the time domain signal limiting defect analysis.
U.S. Pat. No. 5,391,988 discloses a method of detecting a flaw within a conductive object using eddy current testing. A ferrite core has a primary and a secondary coil wound upon it. A rectangular-wave current is supplied to the primary coil to induce a voltage of a pulse wave form in the secondary coil. The pulse duration of the induced voltage is measured at two separate time instances corresponding to two different threshold voltages. The effect of variations in the distance between the ferrite core and the conductive object (lift-off) are cancelled using a comparison of the two measured pulse durations.
U.S. Pat. No. 6,573,712 discloses an inspection device which has a driving section to adjust a position of an inspection probe to maintain a constant lift-off.
Making the right judgement about lift-off occurrence and related lift-off distance enables the removal of the component of the detected signal caused by lift-off that masks the defect signal, so that the defect information may be recovered to provide reliable detection results.